It has long been known in the pulp and paper industry that the manganese in mechanical pulp is likely to have an adverse effect on the bleaching operating, particularly if the bleaching is to be done with a peroxide such as hydrogen peroxide. To remove the manganese and other metals from pulp it is conventional practise to treat the pulp with a chelating agent such as sodium diethylene triamine penta-acetate (DTPA) and then thicken.
From extensive previous investigations, it has been determined that a change of as little as 5 ppm in the manganese at the lower levels of Mn normally required for bleaching, i.e. a reduction of manganese content by 5 ppm can result in a measurable increase in brightening when brightening using peroxide on mechanical pulp.
The use of magnesium sulphate together with peroxide, particularly hydrogen peroxide, to stabilize the hydrogen peroxide in the presence of sodium silicate is well-known for peroxide bleaching and the concept of adding magnesium sulphate to pulp as a stabilizer for peroxide (without sodium silicate) has also been described in literature--see Japanese patent publication 78-44564 published Nov. 30, 1978, inventor Yotsuy, Japanese application 56169890 published Dec. 26, 1981 Mitsubishi Gas and Chemical Inc, which describes a process of refiner bleaching by the addition of magnesium compounds to the chips before the addition of hydrogen peroxide and refining of the chips in the refiner.
Canadian patent 1,249,403 also describes the brightening of high yield or ultra high yield pulps wherein magnesium sulphate is present during a bleaching reaction with hydrogen peroxide.
U.S. Pat. No. 4,731,161 issued Mar. 15, 1988 to Ehrhardt describes the method of bleaching wherein a bleaching solution contains magnesium salts and hydrogen peroxide is used for bleaching chemical pulps.